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Stoichiometry Greek for “measuring elements” The calculations of quantities in
chemical reactions based on a balanced equation.
We can interpret balanced chemical equations several ways.
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In terms of Particles Atom - Element Molecule
• Molecular compound (non- metals)
• or diatomic (O2 etc.) Formula unit
• Ionic Compounds (Metal and non-metal)
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2H2 + O2 2H2O Two molecules of hydrogen and one
molecule of oxygen form two molecules of water.
2 Al2O3 Al + 3O2
2 formula units Al2O3 form 4 atoms Al
and 3 molecules O2
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2Na + 2H2O 2NaOH + H2
2 atoms Na form
2 formula units NaOH and 1 molecule H2
and 2 molecules H2O
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Look at it differently 2H2 + O2 2H2O 2 dozen molecules of hydrogen and 1
dozen molecules of oxygen form 2 dozen molecules of water.
2 x (6.02 x 1023) molecules of hydrogen and 1 x (6.02 x 1023) molecules of oxygen form 2 x (6.02 x 1023) molecules of water.
2 moles of hydrogen and 1 mole of oxygen form 2 moles of water.
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In terms of Moles 2 Al2O3 Al + 3O2
The coefficients tell us how many moles of each kind
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36.04 g reactants36.04 g reactants
In terms of mass The law of conservation of mass applies We can check using moles
2H2 + O2 2H2O
2 moles H2
2.02 g H2
1 moles H2
= 4.04 g H2
1 moles O2
32.00 g O2
1 moles O2
= 32.00 g O2
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In terms of mass 2H2 + O2 2H2O
2 moles H2O18.02 g H2O
1 mole H2O= 36.04 g H2O
2H2 + O2 2H2O
36.04 g (H2 + O2) = 36.04 g H2O
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Mole to mole conversions 2 Al2O3 Al + 3O2
every time we use 2 moles of Al2O3 we make 3 moles of O2
2 moles Al2O3
3 mole O2
or2 moles Al2O3
3 mole O2
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Mole to Mole conversions How many moles of O2 are produced when 3.34 moles of
Al2O3 decompose?
2 Al2O3 Al + 3O2
3.34 moles Al2O3 2 moles Al2O3
3 mole O2 = 5.01 moles O2
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Practice 2C2H2 + 5 O2 4CO2 + 2 H2O
If 3.84 moles of C2H2 are burned, how
many moles of O2 are needed?
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Practice 2C2H2 + 5 O2 4CO2 + 2 H2O
How many moles of C2H2 are needed
to produce 8.95 mole of H2O?
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We can’t measure moles!! What can we do? We can convert grams to moles.
• Periodic Table Then use moles to change chemicals
• Balanced equation Then turn the moles back to grams.
• Periodic table
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Periodic Table
MolesA
MolesB
Massg B
Periodic Table
Balanced Equation
Massg A
•Decide where to start based on the units you are given
•and stop based on what unit you are asked for
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Conversions 2C2H2 + 5 O2 4CO2 + 2 H2O
How many moles of C2H2 are needed
to produce 8.95 g of H2O?
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Conversions 2C2H2 + 5 O2 4CO2 + 2 H2O
If 2.47 moles of C2H2 are burned, how
many g of CO2 are formed?
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For example... If 10.1 g of Fe are added to a solution of
Copper (II) Sulfate, how much solid copper would form?
Fe + CuSO4 Fe2(SO4)3 + Cu
2Fe + 3CuSO4 Fe2(SO4)3 + 3Cu
10.1 g Fe55.85 g Fe1 mol Fe
2 mol Fe3 mol Cu
1 mol Cu63.55 g Cu
= 17.2 g Cu
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To make silicon for computer chips they use this reaction
SiCl4 + 2Mg 2MgCl2 + Si
How many moles of Mg are needed to make 9.3 g of Si?
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To make silicon for computer chips they use this reaction
SiCl4 + 2Mg 2MgCl2 + Si
3.74 g of Mg would make how many moles of Si?
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How do you get good at this?